The present invention is directed to a plug closure and set screw combination for medical implants and, more particularly, to a coaxially nested closure plug and set screw arrangement with individual break-off heads for the plug and screw.
Bone screws are utilized in many types of spinal surgery, such as for osteosynthesis, in order to secure various implants to vertebrae along the spinal column for the purpose of stabilizing and/or adjusting spinal alignment. Although both closed ended and open ended bone screws are known, open ended screws are particularly well suited for connections to rods and connector arms, because such rods or arms do not need to be passed through a closed bore, but rather can be laid or urged into an open receiver channel of such a head.
Typical open ended bone screws include a threaded shank with a pair of parallel projecting branches or arms which form a yoke with a U-shaped slot or channel to receive a rod. Hooks and other types of connectors, as are used in spinal fixation techniques, may also include open ends for receiving rods or projections from other structure. A rod is positioned in the U-shaped channel in generally perpendicular relation to the shank, and the open end of the yoke is closed off by a closure device. The closure device is tightened against the rod to clamp the rod in place against the bottom of the channel. The closure device must positively secure the rod in place to prevent rotational or translational movement of the rod relative to the bone screw and the bone in which it is anchored. Conventional types of closure devices include a threaded plug which is screwed into threads formed into the surfaces forming the U-shaped channel or an outer nut that goes around and is threaded on the arms.
Plug closures are often preferable to nuts because such take up less space. In order to perform the functions required of it, such a plug should be tightly torqued relative to the bone screw to effectively secure the bone screw to the rod and prevent relative rotation or translation. If the rod were straight, this could be more easily accomplished. However, in typical spinal fixation applications, the rod is almost always bent at the location of each bone screw to correctly position the rod for normal or desired curvature of the spinal column. Because the rod is bent, it does not flatly engage the bottom of the groove or U-shaped channel in the head of the bone screw, but tends to be raised from the bottom of the channel at one or both ends. Thus, when a conventional plug is installed, the outer periphery of the lower end of the plug most likely engages parts of the rod that are not set snugly against the floor of the channel opposite the plug. After installation, when the patient""s back is bent during movement activities, the rod may flex slightly relative to the bone screw. Over time, such flexure may allow the rod to move, either translationally or rotationally, causing misalignment of the underlying vertebrae or cause the plug to work loose, because the conventional plug does not lock it securely in place.
What is needed is a plug which is particularly adapted to securely engage and lock in place relative to the bone screw a spinal rod which may be curved at the point of mutual contact.
The present invention provides an improved plug arrangement for securing a spinal fixation rod within an open-headed bone screw and including a cooperating set of a plug and a set screw with individual break-off heads. The bone screw has a threaded shank for screwing into a bone for anchorage to the bone and an open head with arms forming a U-shaped channel to receive a structural member such as a spinal fixation rod. A closure plug engages and is advanced into the open head using cooperating guide and advancement structure on the plug and channel arms, such as cooperating threads, to clamp the rod within the head channel.
Preferably, the guide and advancement structure is also of such a character as to resist splaying tendencies of the arms forming the channel. That is, the plug preferably resists the tendency of the arms to pull away from the plug when torque or force is applied to the plug and which would loosen the plug relative to the bone screw if splaying were allowed to occur. Guide and advancement structure that resists splaying includes, buttress threads, reverse angle threads and helical wound flange structure. The plug is bored and threaded to receive a set screw in a nested relationship within the plug to further secure the rod within the head. In particular, the set screw engages the rod intermediate the perimeter of the plug and where the rod is more likely to engage the bottom of the channel. This also produces multiple points of contact on the rod and cooperatively holds and locks the rod in a desired position relative to the bone screw.
The closure plug and the set screw have respective break-off installation heads to facilitate installation, produce a preferred installation torque and produce a low profile final assembly after the heads are broken off. The installation heads are connected respectively to the plug and set screw by weakened sections which are designed to fail at preselected levels of torque. Thus, the plug and set screw are both set with a preselected degree of tightness against the clamped rod by the level of torque at which the installation heads break off respectively from the plug and set screw.
The closure plug and set screw may include interference devices or formations such as a point, sharpened ring, knurling, at the bottom of each which cut into the rod and enhance the securing engagement of the plug and set screw with the clamped rod. In particular, a set screw may be provided with a centrally positioned set point and/or a xe2x80x9ccup pointxe2x80x9d or annular V-shaped ring at the periphery of the rod engaging end of the set screw. The set point and V-ring operably cuts into the surface of the rod to positively engage the rod to thereby prevent translational and rotational movement of the rod relative to the bone screw. The plug may also be provided with a V-shaped ring to provide for more positive engagement of the plug with the rod. The point and V-rings may be used in combination to better secure the rod, especially when the rod is curved, since the interference devices engage the rod at multiple locations along the rod.
In the great majority of cases, spinal fixation systems are permanently implanted. However, there are situations in which it is necessary to remove, replace, or repair components of an implanted fixation system. For example, such an adjustment might be necessary during implant surgery to adjust the position or alignment of a rod. An adjustment might be necessary after surgery to relieve an undesired pressure or tension which was not detected during surgery. Adjustments might also be required to treat an injury which occurs after the fixation system has been implanted. For these reasons, provisions are made for removal of the plug and set screws of the present invention from the bone screw heads. The set screw of the present invention is provided with structure to enable positive, non-slip engagement of the set screw by a removal tool, such as a pair of diametrically slots which are located at the top face of the set screw and extend partly inward from the periphery, but do not intersect. The closure plug may or may not need separate removal structure. Preferably, the lower part of the set screw deforms on installation so as to interferingly engage the plug, so that the set screw becomes secured to the closure plug in such a manner that rotation of a removal tool engaged with the set screw causes both the set screw and closure plug to be removed substantially as a unit from the head of the bone screw.
Therefore, the objects of the present invention include: providing an improved bone screw arrangement for implantation into a vertebra of a patient; providing such a bone screw arrangement including a threaded bone screw for implanting into a bone such as a vertebra and having an open head formed by a pair of spaced apart arms defining a U-shaped channel to receive a structural member such as a spinal fixation rod; providing such an arrangement including a closure plug which is secured within the channel by a guide and advancement structure such as cooperating threads on the closure plug and inner surfaces forming the channel, to thereby clamp the rod to the bone screw; providing such an arrangement in which the closure plug is bored and tapped to receive a set screw nested therein to further secure the spinal fixation rod within the bone screw head; providing such an arrangement in which both the closure plug and set screw have torque limiting break-off heads so the arrangement has a low profile after final installation and to control the level of torque at which the closure plug and set screw are installed is controlled at a preselected level; providing such an arrangement in which the set screw and also the closure plug have rod surface interference formations thereon for securely engaging the clamped rod against translational and/or rotational movement relative to the bone screw by cutting into the surface of the rod; providing such an arrangement which is particularly effective in securing rods which are curved in the vicinity of the bone screw by engaging the rod at multiple locations; providing such an arrangement in which the set screw has a centrally located set point and a peripherally located V-shaped ring for cutting into the surface of the clamped rod when the set screw is torqued tightly; providing such an arrangement wherein the closure plug has a V-shaped ring formed thereon to cut into the surface of the clamped rod to more securely engage the rod; providing such an arrangement in which the set screw is provided with formations to enable non-slip engagement by a removal tool; providing such an arrangement in which the set screw has a diametrically opposed side slots extending from the periphery partially inward across an outer face for engagement by a removal tool with the side slots being accessible when the head of the set screw breaks away; providing such an arrangement including provisions for removal of the closure plug; providing such an arrangement in which the V-ring of the set screw is formed and positioned in such a manner that the V-ring is deformed into securing engagement with the closure plug by engagement of the V-ring with the surface of the rod; and providing such a nested closure plug and set screw arrangement with break-off heads for clamping a rod to a bone screw which is economical to manufacture, which is compact and secure in use, and which is particularly well adapted for its intended purpose.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.